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The Pore Water Pressure and Settlement Characteristics of Soil Improved by Combined Vacuum and Surcharge Preloading

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The Pore Water Pressure and Settlement Characteristics of Soil Improved by Combined Vacuum and Surcharge Preloading Jie Peng, Wen Guang Ji, Neng Li, Hao Ran Jin 1. Key Laboratory for Ministry of Education for Geo-mechanics and Embankment Engineering, Hohai University, Nanjing, 210098, China 2. Geotechnical Research Institute, Hohai University, Nanjing 210098, China ABSTRACT This study examines the pore water pressure and settlement characteristics of soil improved by combined vacuum and surcharge preloading based on two field tests. It discusses and compares methods of computing settlement and the degree of consolidation between combined vacuum and surcharge preloading and surcharge preloading alone as well. The non- uniform change of in the underground pore water pressure and the change in water depth indicates that the directly effective range of vacuum pumping in this paper should reach 18 m below the surface and that the range of the decline in the pore water pressure during vacuum preloading increases with decreasing depth below the surface. The groundwater table level declines during the process of vacuum preloading, and the restoration of the negative pore water pressure following unloading requires a period of time, as with the dispersion of the positive excess pore water pressure under surcharge preloading. The combination of vacuum and surcharge load can increase both the rate of the soil settlement and the total soil settlement; however, the settlement increment caused by the vacuum load will be less than that caused by the real surcharge load, and the surface settlement of combined vacuum and surcharge preloading is more uniform than that of surcharge preloading. The vacuum load can be equivalent to the positive load when settlement and the degree of consolidation of the combined vacuum and surcharge preloading are calculated and when the settlement correction coefficient (ms) is less than that of the surcharge preloading. KEYWORDS: vacuum combined surcharge preloading, surcharge preloading, field test, pore pressure, settlement INTRODUCTION Recently, the construction of highway embankments over soft clayey deposits has resulted in the advancement of soil improvement techniques. Surcharge preloading is a popular and well- developed method used in practical engineering to improve the shear strength of soft soil and to reduce its post-construction settlement. Surcharge preloading generates positive excess pore water pressure in the soil through applying embankments on the ground. Because of the strong - 1559 - Vol. 18 [2013], Bund. G 1560 permeability of the prefabricated vertical drains(PVDs) inserted into the ground , the pore water pressure in the PVDs remains constant, minimizing the pressure differences produced between the soil mass and the PVDs; this effect causes the pore water to discharge from the soil mass and increases the rate of soil consolidation, thereby reinforcing the soil. Surcharge preloading is widely used in soft soil improvement (1-4). The primary disadvantages of surcharge preloading are that it requires a long preloading time and large quantity of embankment material and that it has an accompanying instability problem. Surcharge preloading can be combined with vacuum preloading to reduce the quantity of fill material required, to accelerate the rate of consolidation, to shorten construction periods and to decrease the problem of embankment instability. Vacuum preloading, originally introduced by W. Kjellman(5), decreases the pressures below the sealing membrane and within the PVDs caused by vacuum pumping. Because of the weak permeability of the soil, the rate of decrease in pore water pressure in the soil is slower than that in the PVDs, and differences in pressure developed between the soil mass and the PVDs. The pressure differentials drive the pore water from the soil to the PVDs, leading to a decrease in the underground pore water pressure, while the total stress is maintained at the same level. As a result, the effective stress of the soil is increased to accelerate consolidation (6). Following this principle, many scholars have studied the consolidation mechanism, calculation methods and construction technologies of vacuum preloading (7-13). The effectiveness of combined vacuum and surcharge preloading has been discussed by Chai et al. (2) and Indraratna et al. (14). In this method, the vacuum pressure can be distributed to a greater depth in the subsoil, and the consolidation time of stage construction can be minimized (10). Moreover, the rate of embankment construction can be increased (15). The post-construction settlement will be significantly less with increasing soil stiffness and shear strength owing to consolidation, thereby eliminating the risk of differential soil settlement (16). Previous research regarding the soil pore water pressure characteristics of soil improved by combined vacuum and surcharge preloading is still insufficient, and comparative field test studies of the deformation characteristics between combined vacuum and surcharge preloading and surcharge preloading alone are rare. With two field tests, this paper examines the pore water pressure and soil settlement characteristics of soil improved by combined vacuum and surcharge preloading in similar geological sites; this paper also discusses and compares the methods of computing settlement and the degree of consolidation for these two methods. K32+475 1 3.50 2 23.50 3 27.50 4 1 Loam 2 M uddy loam 3 Loam 4 Pebbly clay Figure 1: Geological section map of Test A Vol. 18 [2013], Bund. G 1561 K23+880 K23+720 K23+655 K23+597 0.00 1 1 1 3.10 1 2 3.70 2 4.00 5.50 5.70 5.30 3 3 3 14.80 14.60 3 4 4 19.30 18.50 21.20 6 5 4 23.00 5 23.30 24.60 5 26.00 26.10 7 27.30 7 7 30.10 30.60 6 33.78 32.80 8 8 8 8 1 Cultivated soil 2 Shell bed 3 Muddy clay 4 Silt 5 Silt 6 Clay 7 Muddy Clay 8 Pebble Figure 2: Geological section map of Test B OVERVIEW OF FIELD TESTS The two field tests discussed in this paper are conducted in Hangzhou city, Zhejiang province, China (hereinafter referred to as Test A) and in Jiangmen city, Guangdong province, China (hereinafter referred to as Test B). Project profile of Test A Hangzhou-Jinhua-Quzhou Expressway is an important artery in the expressway network of Zhejiang province in China. Soft soil is common in Hangzhou city, the provincial capital of Zhejiang. In this area, the embankment is high and deep, beneath which soft soil is located; therefore, the combined vacuum and surcharge preloading method is used to treat the soft soil in this area. The section of Test A examined in this paper was named as the VS-A-1 section. The geological section in this section and the properties of each soil layer are illustrated in Figure 1 and Table 1. A portion of the construction parameters in this section is listed in Table 2. The loading curves are shown in Figure 3. The monitoring instruments and their positions in this cross section can be observed in Figure 5. Vol. 18 [2013], Bund. G 1562 160 140 120 100 Surcharge Load 80 Vacuum Load 60 Load(kPa) 40 20 0 0 100 200 300 400 500 600 700 Time(days) Figure 3: Loading curves of test A 200 180 160 140 120 100 Surcharge Load(VS-B-2) 80 Surcharge Load(VS-B-1) Load(kPa) 60 Surcharge Load(S-B-2) 40 Surcharge Load(S-B-1) Vacuum Load(VS-B-2) 20 Vacuum Load(VS-B-1) 0 0 100 200 300 400 500 600 700 Time(days) Figure 4: Loading curves of test B Project profile of Test B Zhongshan-Jiangmen Expressway is an important part of the expressway network of Guangdong province in China; the soft soil in the section K23+565.3-K23+987.8 is thick (approximately 21-31 m in thickness), and the embankment fill is higher (6.7-7.8 m in fill height). In this area, surcharge preloading and combined vacuum and surcharge preloading are both adopted to treat the soft ground. The geological section and soil layer distribution for each cross-section are described in Figure 2. The basic parameters in each soil layer are shown in the Table 1. The treatment range, construction parameters in the corresponding parts and the sections studied with these two Vol. 18 [2013], Bund. G 1563 methods are listed in Table 2. For the sake of simplicity and clarity, the surcharge preloading sections, K23 + 597 and K23 + 655, are hereafter referred to as S-B-1 and S-B-2, respectively, and the combined vacuum and surcharge preloading sections, K23 + 720 and K23 + 880, are hereafter referred to as VS-B-1 and VS-B-2, respectively. Table 1: Geological parameters of two field tests (average value) Water Unit Liquid Plastic -3 Field content weight Void limit limit av Cv (×10 Soil layer -1 2 test ω 0 ratio e0 WL WP (Mpa ) cm /s) (%) (kN/m3) (%) (%) 1. lloam 28.5 17.5 0.796 20.4 34.3 0.370 5.20 A 2. muddy loam 40.0 18.1 1.040 30.7 43.2 0.671 2.50 3. loam 32.5 18.0 0.902 21.9 35.1 0.342 3.20 4. pebbly clay 18.5 17.8 0.589 - - 0.180 4.50 1. cultivated soil 39.7 18.0 1.056 - - 0.350 8.60 2. shell/oyster 38.9 18.4 1.001 - - 0.150 30.40 3. muddy clay 54.3 18.0 1.271 23.6 38.9 0.921 2.07 B 4. silt 56.6 18.2 1.280 24.9 45.2 0.621 2.90 5. silt 45.2 18.2 1.056 27.4 41.4 0.150 40.00 6.
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